1. Field of the Invention
The present invention relates to a piezoelectric ceramic composition for use in a ceramic filter, a ceramic oscillator, a piezoelectric transducer, a variety of ceramic sensors, a ceramic buzzer or the like. The present invention also relates to a piezoelectric device such as a piezoelectric ceramic oscillator and a piezoelectric ceramic filter including a capacitive part and a piezoelectric oscillating part using the piezoelectric ceramic composition.
2. Description of the Prior Art
Conventional examples of a piezoelectric ceramic material include so-called PT ceramics comprising PbTiO.sub.3 as a main component, so-called PZT ceramics comprising Pb(Ti, Zr)O.sub.3 as a main component, and a multi-component piezoelectric ceramic composition comprising several types of composite perovskite compositions such as Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 and Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3 in the form of a solid solution. The use of these compositions provides a wide range of piezoelectric ceramics varying in the characteristics by selecting a composition ratio of the components suitably to meet a purpose of use. For example, Pb(Zn.sub.1/3 Nb.sub.2/3).sub.A (Sn.sub.1/3 Nb.sub.2/3).sub.B Ti.sub.C Zr.sub.D O.sub.3 composition (disclosed in Japanese Patent Publication (Tokko-sho) Nos. 52-17239 and 51-7318) and Pb(Sn.sub.a Sb.sub.1-a).sub.X Ti.sub.Y Zr.sub.Z O.sub.3 composition (disclosed in Japanese Patent Publication (Tokko-sho) Nos. 54-32516 and 54-36757) are suitable for use in a high frequency ceramic oscillator or filter, because of their excellent piezoelectricity and small size of crystal grains. Therefore, these piezoelectric ceramic compositions are used in ceramic filters, ceramic oscillators, piezoelectric transducers, ceramic sensors or the like.
However, there is a problem when such piezoelectric ceramics are heated to a temperature of about 250.degree. C. for, for example, solder reflow and cooled to room temperature. In this case, the piezoelectric properties such as resonance frequency of the piezoelectric ceramics before the heat treatment are different from those after the heat treatment.
The heat treatment changes the piezoelectric properties including a mechanical quality factor (Q.sub.m), an electromechanical coupling coefficient (k.sub.t) and a dielectric constant (.epsilon./.epsilon..sub.0) of the conventional piezoelectric ceramic compositions. Thus, the conventional piezoelectric ceramic composition has poor heat resistance. In addition, the piezoelectric properties drift due to heat. This change due to thermal shock may lead to a change in the characteristics of the frequency dependence of a filter produced with such piezoelectric ceramics. Japanese Laid-Open Patent Publication (Tokkai-Hei) Nos. 8-239269 and 9-142930 disclose materials that are not affected by heat. More specifically, a composite oxide of Y, Nb or the like is allowed to be a component of the main component of PZT, or Cr is added thereto, so that the resistance of the piezoelectric ceramics becomes low.
When an electronic component such as a ceramic filter and a ceramic oscillator is fabricated as a chip, a chip element is mounted with solder at a higher temperature. The temperature of the element may be about 200.degree. C. When a piezoelectric device formed of a conventional piezoelectric ceramic composition is heated at 150.degree. C. for 1 hour, the resonance frequency fr changes by several % immediately after the heat treatment, when compared to that before the heat treatment. After the heat treatment, the resonance frequency fr changes over time and drifts to a value different from that before the heat treatment. In other words, the characteristics of the conventional piezoelectric ceramic composition change because of poor heat resistance, and drift occurs due to heat. Therefore, the piezoelectric device has low reliability and disadvantages in mass production.